In conventional typical press brakes, as shown in FIG. 7, a lower die 101 and an upper die 103 are attached to the top surface of a table 100 and the bottom of a not-shown ram via a holder 102, respectively, so that they are opposed to each other. A plate-like work W is supported on the lower die 101. The upper die 103 is lowered toward this work W together with the ram. The upper die 103 comes into contact with the work W on the way down, and the work W undergoes a pressing force from the upper die 103. The work W is pushed into a V-shaped groove 104 of the lower die 101 by this pressing force, being bent into a bending angle a corresponding to the amount of pushing d.
Among press brakes of this type are a press brake of hydraulic drive system in which a hydraulic cylinder is used as the driving source of the ram, and a press brake of motor drive system in which a servo motor is used. The press brake of motor drive system is equipped with a reciprocating drive mechanism which is configured so that the servo motor is connected with a ball screw mechanism through a power transmission mechanism. In this reciprocating drive mechanism, the number of rotations and the direction of rotation of the servo motor are controlled to move up and down the ram via the ball screw mechanism.
In recent years, improved performance has been strongly demanded of press brakes. To accelerate the bending operation and to enhance the precision of the bending process, it has been practiced to move the ram at low speed during the bending process while moving down or up the ram at high speed at approach time before the bending process and at return time after the bending process.
When the up-and-down movements of the ram are controlled as described above, it is possible to satisfy productivity requirements by the high speed operation of the ram at approach time and at return time. In the bending process, on the other hand, the ram is moved at low speed to bend the work gradually. As a result, it is possible to prevent the work from warpage and the like, and secure operator's safety.
By the way, the servo motor has a maximum number of rotations N and a maximum torque T available. The maximum speed of the ram and the maximum pressing force from the dies are determined by the capability of the servo motor and a reduction ratio of the power transmission mechanism.
Assume here that Nb is the number of rotations of the ball screw mechanism when moving up and down the ram at a desired maximum speed. Then, the reduction ratio (1/A) of the power transmission mechanism is given by 1/A=Nb/N, and the maximum pressing torque Tp of the press brake is given by Tp=A·T=T·N/Nb.
As can be seen from above, given the number of rotations Nb of the ball screw mechanism in moving up and down the ram at a desired maximum speed, the pressing capability of the press brake is determined uniquely.
In the press brake of motor drive system described above, controls for switching the number of rotations of the servo motor between two levels, or high speed and low speed, are exercised so as to move the ram at low speed during a bending process while moving it at high speed at approach time and at return time. In this case, the possible output torque of the servo motor is constant or varies only slightly even when the number of rotations is changed.
Nevertheless, when the reduction ratio (1/A) is set lower in order to increase the moving speed of the ram at approach time and at return time, the maximum pressing torque Tp tends to be short. On the other hand, when the reduction ratio (1/A) is set higher in order to provide a sufficiently high maximum pressing torque Tp, it is impossible to increase the moving speed of the ram at approach time and at return time sufficiently.
To solve this problem, the applicant has recently developed a press brake which has a reciprocating drive mechanism including a first power transmission mechanism for reducing motor rotation at a first reduction ratio and transmitting it to the ball screw mechanism, and a second power transmission mechanism for reducing motor rotation at a second reduction ratio higher than the first reduction ratio and transmitting it to the ball screw mechanism.
In this press brake, at approach time, the first power transmission mechanism and the second power transmission mechanism are set to an active state and an inactive state, respectively, so that the die is moved toward the work at high speed. Then, the first power transmission mechanism and the second power transmission mechanism are set to an inactive state and an active state, respectively, so that the die is moved at low speed to bend the work. Moreover, at return time, the first power transmission mechanism and the second power transmission mechanism are set to an active state and an inactive state, respectively, so that the die is moved in the direction away from the work at high speed.
Nevertheless, the first and second power transmission mechanisms each comprise a number of pulleys and belts, as well as two electromagnetic clutches for switching the power transmission paths. This complicates the mechanisms and increases the parts count. In particular, the second power transmission mechanism has an intermediate shaft arranged between the output shaft of the servo motor and a feed screw of the ball screw mechanism, and transmits rotation via an intermediate pulley on the intermediate shaft. This produces the problem of high inertia with poor response characteristics.
There has recently been proposed a press machine comprising a reciprocating drive mechanism which uses two motors as rotational driving sources. The rotation of a first motor is converted into high-speed low-torque rotation by a reducing device having a lower reduction ratio, and transmitted to a male screw of a male-female screw mechanism. The rotation of a second motor is converted into low-speed high-torque rotation by a reducing device having a higher reduction ratio, and transmitted to a female screw of the male-female screw mechanism (see Publication of Japanese Patent No. 3344721).
According to this reciprocating drive mechanism, the power transmission mechanisms are simplified in configuration, with a reduction in parts count. To prevent the male screw from rotating together with the rotation of the female screw, the male-female screw mechanism requires a braking device, and a control device for controlling the operation of the braking device as well. The reciprocating drive mechanism thus has a large-scale structure as a whole. In addition, since the braking device needs to brake the maximum torque to act on the male screw, the braking device itself becomes greater. This can increase the inertia and make it difficult to achieve favorable response characteristics.
It is an object of this invention to provide a reciprocating drive mechanism which can move a shaping mechanism or the like at low speed to allow a sufficiently high maximum pressing torque during processes under pressure and can also move the shaping mechanism or the like at high speed at approach time and at return time, and a press machine using the mechanism.
Moreover, it is another object of this invention to provide a reciprocating drive mechanism which has simple configuration, a small parts count, small inertia, and favorable response characteristics, and requires no braking device for preventing corotation nor control device thereof to be provided and installed additionally, and a press machine using the mechanism.